Magicka regen outside of combat is unaffected by the Atronach ever since the mundus was buffed in Stonethorn. I thought things felt a bit off on the PTS, but I didn't think much of it — however after playing on live for a while now, my regen out of combat feels substantially lower than it did last patch.
Harven's Extended Stats addon shows that applying the mundus does not affect Idle magicka regen at all. I drained my magicka and observed how quickly it regenerated, and with all divines the out of combat regen is definitely ~500 less then it should be.
Without Atronach:
With Atronach:
[EDIT] After further testing, I can confirm that this is also the case with the Serpent mundus — it does not work outside of combat either.
Edited by Langeston on September 2, 2020 9:29PM